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Group 2 – FARM | Poster Abstracts
critical slip of the linear slip-weakening formulation. The results show that there are trade-offs
between the parameters of the rupture initiation procedure and the properties of interface friction,
underscore the need to quantify experimental parameters for proper interpretation of the
experiments, and highlight the importance of rupture initiation in simulations of both experiments
and real-life earthquake events.
2-086
SHEAR HEATING-INDUCED THERMAL PRESSURIZATION DURING
EARTHQUAKE NUCLEATION Schmitt SV, Segall P, and Matsuzawa T
Shear heating-induced thermal pressurization has long been posited as a weakening mechanism
during earthquakes. It is often assumed that thermal pressurization does not become important
until earthquakes become moderate to large in magnitude. Segall & Rice [2006, JGR], however,
suggested that thermal effects may become dominant during the quasi-static nucleation phase, well
before the onset of seismic radiation. Using the slip evolution given by rate- and state-dependent
friction--along with reasonable estimates of heat and pore pressure transport parameters--they
estimated that thermal pressurization dominates weakening at slip rates in excess of 10^{-5} to
10^{-3} m/s.
We further explore this problem numerically, assuming a fault in a 2D elastic medium and
accounting for full thermomechanical coupling. The fault is governed by rate and state friction with
the radiation damping approximation to simulate inertial effects. Thermal diffusion is computed
via finite differences on a grid that adaptively remeshes to minimize computational expense while
maintaining accuracy. To start, we neglect fault zone thickness and model the fault as a plane. This
approximation is valid for times much greater than the diffusion time across the fault zone. With
uniform transport properties, it leads to a direct relationship between pore pressure on the fault
and temperature [Rice, 2006, JGR], thus requiring only one finite difference grid.
Our results thus far indicate that thermal pressurization does in fact dominate at modest slip
speeds that are slightly lower than those estimated by Segall & Rice [2006]. Interestingly, the
thermal pressurization process leads to a contraction of the nucleation zone, rather than the
growing crack (aging law) or unidirectional slip pulse (slip law) associated with drained rate- and
state-dependent frictional nucleation.
If allowed to proceed to higher--yet still quasi-static--slip speeds, our modeled nucleation zone
continues to shrink nearly to zero width. We believe this is a consequence of treating the fault as a
planar surface rather than a finite-width shear zone. Such an approximation overestimates the fault
temperature at higher slip speeds, when time steps are no longer much greater than the diffusion
time across the width of the shear zone. Our current work is to include the finite fault thickness, so
that we may conduct simulations up to speeds at which seismic radiation becomes significant.
2-087
A COMPARISON OF DAMAGE ZONE DECAY AROUND SMALL AND LARGE
FAULTS Savage HM, Brodsky EE, and Johns M
The increase in fracture density around faults is a useful proxy for assessing the stress field when
the damage formed. However, the damage surrounding large faults will inevitably be a function of
the superposition of several main fault strands, in addition to farther-flung secondary faults, that
form over many episodes of slip. To investigate the decay of fracture density away from a single
fault with presumably few episodes of slip, we are measuring damage around small displacement
faults at Four Mile Beach, Santa Cruz, CA, that are isolated or have few discrete strands. We
2008 SCEC Annual Meeting | 187